1. Field of the Invention
The present invention relates generally to an apparatus for controlling the electric current supplied to a heating element and, more particularly, to such an apparatus for use in an electric kiln adapted to fire ceramic ware.
2. Description of the Prior Art
There has recently been a substantial increase in popularity in the home manufacture, either as a hobby or in a small home business, of heat fired ceramic ware. Consequently, a large market has developed for the sale of electric furnaces and kilns to the ceramic hobbyist and the like. One problem often associated with the use of such kilns is that the ceramic materials customarily used have a high thermal shock intolerance at low temperatures. Thus, the temperature in the kiln must be very gradually increased or the ceramic material being fired will fracture. This has often required that the operator be close to the kiln over the entire heating cycle so that he could manually increase the temperature at a gradual rate. The inconvenience to the operator associated with such a procedure is apparent.
Several devices have been proposed in the past to automatically increase the temperature in the kiln at a gradual rate to relieve the operator of the need to manually attend to the kiln. In general, however, such devices utilize relatively expensive electronic or mechanical components which make their price prohibitively high for home use. For example, U.S. Pat. Nos. 3,814,902 to Fann and 3,315,891 to Allen disclose heat control devices comprising a periodic on-off controller which supplies current to the heating element of the kiln. A thermocouple for sensing the temperature inside the kiln is connected to the controller. Both Allen and Fann supra, utilize additional complex circuitry which produces a signal that is combined with that of the thermocouple to fool the controller into sensing an initial temperature in the kiln that is higher than the actual temperature so that the controller does not provide a high heat output at the beginning of the heating cycle. Gradually the additional signal produced by the circuitry diminishes to zero to provide a gradual increase in the temperature in the kiln. A disadvantage of these devices is the need for relatively expensive timing circuits, variable potentiometers and other electrical elements.
U.S. Pat. No. 3,855,452 to Flasza et al discloses another control system for a ceramic kiln which again utilizes quite expensive solid-state timing circuits and logic blocks. Moreover, the use of such solid-state circuits in a control system mounted on or near a ceramic kiln is disadvantageous since such circuits are temperature sensitive and may degrade under the operating temperatures achieved by the kiln. U.S. Pat. No. 1,861,472 to Glitzke discloses a device for gradually increasing the temperature in a furnace comprising a clock motor which progressively cuts out sections of a variable resistance element to thereby increase the electric current being supplied to the heating element. However, such a variable resistance element is often expensive and prone to failure.